1. Field of the Invention
The invention relates to a steering system that assists a driver's vehicle traveling by applying an assist force of a motor to a steering mechanism.
2. Description of Related Art
An electric power steering system of this type performs what is called lane keeping assist control in which an assist force from a motor is applied to a steering mechanism so that a vehicle can easily kept traveling in a lane in which the vehicle is traveling (see Japanese Patent Application Publication No. 2012-232704 (JP 2012-232704 A)). The electric power steering system described in JP 2012-232704 A includes an imaging apparatus, a motor, and a control apparatus. The imaging apparatus images an area ahead of a vehicle. The motor applies an assist force to the steering mechanism of the vehicle. The control apparatus controls driving of the motor. The control apparatus acquires information on a lane in the traveling direction of the vehicle from image data on the area ahead of the vehicle acquired by the imaging apparatus. The control apparatus then sets a target traveling line based on the lane information acquired. When the actual traveling line of the vehicle deviates from the target traveling line, the control apparatus controls the driving of the motor so as to apply, to the steering mechanism, such an assist force that recovers the actual traveling line of the vehicle to the target traveling line.
A frictional force, an inertia force, and the like generated in the steering mechanism when the steering mechanism is driven vary with vehicles. Thus, for two vehicles between which the frictional force, the inertia force, and the like generated in the steering mechanism vary, the assist force actually applied to the steering mechanism varies between the vehicles even when the driving control of the motor is similarly performed. The variation in assist force in turn varies the response of lane keeping assist control, making the driver feel uncomfortable.
The above-described problem is not limited to the steering system that performs the lane keeping assist control. The problem is common to steering systems that performs various types of driving assist control that assist the driver's vehicle traveling by applying the assist force of the motor to the steering mechanism.